Subject: Re: [PATCH net v3] net: gso: Forbid IPv6 TSO with extensions on devices with only IPV6_CSUM

From: Willem de Bruijn <[email-masked]>
Date: Fri, 20 Mar 2026 15:03:53 -0400

xietangxin wrote:
> 
> 
> On 3/15/2026 12:19 AM, Willem de Bruijn wrote:
> > Paolo Abeni wrote:
> >> On 3/6/26 7:32 AM, xietangxin wrote:
> >>> On 3/5/2026 11:21 PM, Paolo Abeni wrote:
> >>>> On 3/5/26 3:57 PM, Willem de Bruijn wrote:
> >>>>> xietangxin wrote:
> >>>>>> 在 2025/8/14 18:51, Jakub Ramaseuski 写道:
> >>>>>>> When performing Generic Segmentation Offload (GSO) on an IPv6 packet that
> >>>>>>> contains extension headers, the kernel incorrectly requests checksum offload
> >>>>>>> if the egress device only advertises NETIF_F_IPV6_CSUM feature, which has 
> >>>>>>> a strict contract: it supports checksum offload only for plain TCP or UDP 
> >>>>>>> over IPv6 and explicitly does not support packets with extension headers.
> >>>>>>> The current GSO logic violates this contract by failing to disable the feature
> >>>>>>> for packets with extension headers, such as those used in GREoIPv6 tunnels.
> >>>>>>>
> >>>>>>> This violation results in the device being asked to perform an operation
> >>>>>>> it cannot support, leading to a `skb_warn_bad_offload` warning and a collapse
> >>>>>>> of network throughput. While device TSO/USO is correctly bypassed in favor
> >>>>>>> of software GSO for these packets, the GSO stack must be explicitly told not 
> >>>>>>> to request checksum offload.
> >>>>>>>
> >>>>>>> Mask NETIF_F_IPV6_CSUM, NETIF_F_TSO6 and NETIF_F_GSO_UDP_L4
> >>>>>>> in gso_features_check if the IPv6 header contains extension headers to compute
> >>>>>>> checksum in software.
> >>>>>>>
> >>>>>>> The exception is a BIG TCP extension, which, as stated in commit
> >>>>>>> 68e068cabd2c6c53 ("net: reenable NETIF_F_IPV6_CSUM offload for BIG TCP packets"):
> >>>>>>> "The feature is only enabled on devices that support BIG TCP TSO.
> >>>>>>> The header is only present for PF_PACKET taps like tcpdump,
> >>>>>>> and not transmitted by physical devices."
> >>>>>>>
> >>>>>>> kernel log output (truncated):
> >>>>>>> WARNING: CPU: 1 PID: 5273 at net/core/dev.c:3535 skb_warn_bad_offload+0x81/0x140
> >>>>>>> ...
> >>>>>>> Call Trace:
> >>>>>>>  <TASK>
> >>>>>>>  skb_checksum_help+0x12a/0x1f0
> >>>>>>>  validate_xmit_skb+0x1a3/0x2d0
> >>>>>>>  validate_xmit_skb_list+0x4f/0x80
> >>>>>>>  sch_direct_xmit+0x1a2/0x380
> >>>>>>>  __dev_xmit_skb+0x242/0x670
> >>>>>>>  __dev_queue_xmit+0x3fc/0x7f0
> >>>>>>>  ip6_finish_output2+0x25e/0x5d0
> >>>>>>>  ip6_finish_output+0x1fc/0x3f0
> >>>>>>>  ip6_tnl_xmit+0x608/0xc00 [ip6_tunnel]
> >>>>>>>  ip6gre_tunnel_xmit+0x1c0/0x390 [ip6_gre]
> >>>>>>>  dev_hard_start_xmit+0x63/0x1c0
> >>>>>>>  __dev_queue_xmit+0x6d0/0x7f0
> >>>>>>>  ip6_finish_output2+0x214/0x5d0
> >>>>>>>  ip6_finish_output+0x1fc/0x3f0
> >>>>>>>  ip6_xmit+0x2ca/0x6f0
> >>>>>>>  ip6_finish_output+0x1fc/0x3f0
> >>>>>>>  ip6_xmit+0x2ca/0x6f0
> >>>>>>>  inet6_csk_xmit+0xeb/0x150
> >>>>>>>  __tcp_transmit_skb+0x555/0xa80
> >>>>>>>  tcp_write_xmit+0x32a/0xe90
> >>>>>>>  tcp_sendmsg_locked+0x437/0x1110
> >>>>>>>  tcp_sendmsg+0x2f/0x50
> >>>>>>> ...
> >>>>>>> skb linear:   00000000: e4 3d 1a 7d ec 30 e4 3d 1a 7e 5d 90 86 dd 60 0e
> >>>>>>> skb linear:   00000010: 00 0a 1b 34 3c 40 20 11 00 00 00 00 00 00 00 00
> >>>>>>> skb linear:   00000020: 00 00 00 00 00 12 20 11 00 00 00 00 00 00 00 00
> >>>>>>> skb linear:   00000030: 00 00 00 00 00 11 2f 00 04 01 04 01 01 00 00 00
> >>>>>>> skb linear:   00000040: 86 dd 60 0e 00 0a 1b 00 06 40 20 23 00 00 00 00
> >>>>>>> skb linear:   00000050: 00 00 00 00 00 00 00 00 00 12 20 23 00 00 00 00
> >>>>>>> skb linear:   00000060: 00 00 00 00 00 00 00 00 00 11 bf 96 14 51 13 f9
> >>>>>>> skb linear:   00000070: ae 27 a0 a8 2b e3 80 18 00 40 5b 6f 00 00 01 01
> >>>>>>> skb linear:   00000080: 08 0a 42 d4 50 d5 4b 70 f8 1a
> >>>>>>>
> >>>>>>> Fixes: 04c20a9356f283da ("net: skip offload for NETIF_F_IPV6_CSUM if ipv6 header contains extension")
> >>>>>>> Reported-by: Tianhao Zhao <[email-masked]>
> >>>>>>> Suggested-by: Michal Schmidt <[email-masked]>
> >>>>>>> Suggested-by: Willem de Bruijn <[email-masked]>
> >>>>>>> Signed-off-by: Jakub Ramaseuski <[email-masked]>
> >>>>>>> ---
> >>>>>>> ---
> >>>>>>>  net/core/dev.c | 12 ++++++++++++
> >>>>>>>  1 file changed, 12 insertions(+)
> >>>>>>>
> >>>>>>> diff --git a/net/core/dev.c b/net/core/dev.c
> >>>>>>> index b28ce68830b2b..1d8a4d1da911e 100644
> >>>>>>> --- a/net/core/dev.c
> >>>>>>> +++ b/net/core/dev.c
> >>>>>>> @@ -3778,6 +3778,18 @@ static netdev_features_t gso_features_check(const struct sk_buff *skb,
> >>>>>>>  		if (!(iph->frag_off & htons(IP_DF)))
> >>>>>>>  			features &= ~NETIF_F_TSO_MANGLEID;
> >>>>>>>  	}
> >>>>>>> +
> >>>>>>> +	/* NETIF_F_IPV6_CSUM does not support IPv6 extension headers,
> >>>>>>> +	 * so neither does TSO that depends on it.
> >>>>>>> +	 */
> >>>>>>> +	if (features & NETIF_F_IPV6_CSUM &&
> >>>>>>> +	    (skb_shinfo(skb)->gso_type & SKB_GSO_TCPV6 ||
> >>>>>>> +	     (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4 &&
> >>>>>>> +	      vlan_get_protocol(skb) == htons(ETH_P_IPV6))) &&
> >>>>>>> +	    skb_transport_header_was_set(skb) &&
> >>>>>>> +	    skb_network_header_len(skb) != sizeof(struct ipv6hdr) &&
> >>>>>>> +	    !ipv6_has_hopopt_jumbo(skb))
> >>>>>>> +		features &= ~(NETIF_F_IPV6_CSUM | NETIF_F_TSO6 | NETIF_F_GSO_UDP_L4);
> >>>>>>>  
> >>>>>>>  	return features;
> >>>>>>>  }
> >>>>>> question about this patch affecting tunneled IPv6-in-IPv4 packets
> >>>>>>
> >>>>>> In our environment with a hinic NIC, we use VXLAN tunnels where
> >>>>>> the outer header is IPv4 and the inner is IPv6. After this commit,
> >>>>>> large packets no longer use hardware TSO and fall back to software segmentation.
> >>>>>>
> >>>>>> In the VXLAN IPv6-in-IPv4 case, `skb_shinfo(skb)->gso_type` includes
> >>>>>> `SKB_GSO_TCPV6` (inner is IPv6 TCP), but the network header points to the outer
> >>>>>> IPv4 header. Thus `skb_network_header_len(skb)` returns the IPv4 header length
> >>>>>> (usually 20), which is not equal to `sizeof(struct ipv6hdr)` (40). This causes
> >>>>>> the condition to trigger and clears `NETIF_F_TSO6`, even though the inner IPv6
> >>>>>> packet has no extension headers and the device is capable of handling TSO for
> >>>>>> such packets.
> >>>>>>
> >>>>>> Is it the intended behavior to disable TSO for all tunneled IPv6-in-IPv4 packets
> >>>>>> when the NIC lacks NETIF_F_HW_CSUM, even if the inner IPv6 header has no extensions?
> >>>>>>
> >>>>>> Any feedback or guidance would be greatly appreciated.
> >>>>>
> >>>>> That is definitely unintended.
> >>>>>
> >>>>> Thanks for the clear analysis.
> >>>>>
> >>>>> I was about to write a refinement that might catch this case,
> >>>>> something like
> >>>>>
> >>>>> @@ -3819,8 +3819,10 @@ static netdev_features_t gso_features_check(const struct sk_buff *skb,
> >>>>>             (skb_shinfo(skb)->gso_type & SKB_GSO_TCPV6 ||
> >>>>>              (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4 &&
> >>>>>               vlan_get_protocol(skb) == htons(ETH_P_IPV6))) &&
> >>>>> -           skb_transport_header_was_set(skb) &&
> >>>>> -           skb_network_header_len(skb) != sizeof(struct ipv6hdr))
> >>>>> +             ((!skb->encapsulation &&
> >>>>> +               skb_transport_header_was_set(skb) &&
> >>>>> +               skb_network_header_len(skb) != sizeof(struct ipv6hdr)) ||
> >>>>> +              (skb_inner_network_header_len(skb) != sizeof(struct ipv6hdr))))
> >>>>>                 features &= ~(NETIF_F_IPV6_CSUM | NETIF_F_TSO6 | NETIF_F_GSO_UDP_L4);
> >>>>>
> >>>>> But, how are these VXLAN IPv6-in-IPv4 packets having
> >>>>> vlan_get_protocol(skb) == htons(ETH_P_IPV6)?
> >>>>>
> >>>>> Shouldn't that be the protocol of the outer headr, so ETH_P_IP, and
> >>>>> thus this branch not reached at all? (Which itself would leave a false
> >>>>> positive as now an inner network header with extensions would not be
> >>>>> caught..)
> >>>>
> >>>> Also the tunnel could have ENCAP_TYPE_IPPROTO, and likely we need to
> >>>> disable csum even in that case? Possibly something alike the following
> >>>> could work?
> >>>>
> >>>> Side note, I *think* that replacing SKB_GSO_UDP_L4 with separate
> >>>> SKB_GSO_UDPV4_L4 SKB_GSO_UDPV6_L4 would remove a bit of complexity in
> >>>> serveral places, but I'm not sure how much invasive would be such a change.
> >>>>
> >>>> ---
> >>>> diff --git a/net/core/dev.c b/net/core/dev.c
> >>>> index 4af4cf2d63a4..f9824dfef376 100644
> >>>> --- a/net/core/dev.c
> >>>> +++ b/net/core/dev.c
> >>>> @@ -3769,6 +3769,22 @@ static netdev_features_t
> >>>> dflt_features_check(struct sk_buff *skb,
> >>>>  	return vlan_features_check(skb, features);
> >>>>  }
> >>>>
> >>>> +static bool skb_gso_has_extension_hdr(const struct sk_buff *skb)
> >>>> +{
> >>>> +	if (!skb->encapsulation)
> >>>> +		return ((skb_shinfo(skb)->gso_type & SKB_GSO_TCPV6 ||
> >>>> +			(skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4 &&
> >>>> +			 vlan_get_protocol(skb) == htons(ETH_P_IPV6))) &&
> >>>> +			skb_transport_header_was_set(skb) &&
> >>>> +			skb_network_header_len(skb) != sizeof(struct ipv6hdr));
> >>>> +
> >>>> +	return (skb->inner_protocol_type == ENCAP_TYPE_IPPROTO ||
> >>>> +		((skb_shinfo(skb)->gso_type & SKB_GSO_TCPV6 ||
> >>>> +		  (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4 &&
> >>>> +		   inner_ip_hdr(skb)->version == 6)) &&
> >>>> +		 skb_inner_network_header_len(skb) != sizeof(struct ipv6hdr)));
> >>>> +}
> >>>> +
> >>>>  static netdev_features_t gso_features_check(const struct sk_buff *skb,
> >>>>  					    struct net_device *dev,
> >>>>  					    netdev_features_t features)
> >>>> @@ -3815,12 +3831,7 @@ static netdev_features_t gso_features_check(const
> >>>> struct sk_buff *skb,
> >>>>  	/* NETIF_F_IPV6_CSUM does not support IPv6 extension headers,
> >>>>  	 * so neither does TSO that depends on it.
> >>>>  	 */
> >>>> -	if (features & NETIF_F_IPV6_CSUM &&
> >>>> -	    (skb_shinfo(skb)->gso_type & SKB_GSO_TCPV6 ||
> >>>> -	     (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4 &&
> >>>> -	      vlan_get_protocol(skb) == htons(ETH_P_IPV6))) &&
> >>>> -	    skb_transport_header_was_set(skb) &&
> >>>> -	    skb_network_header_len(skb) != sizeof(struct ipv6hdr))
> >>>> +	if (features & NETIF_F_IPV6_CSUM && skb_gso_has_extension_hdr(skb))
> >>>>  		features &= ~(NETIF_F_IPV6_CSUM | NETIF_F_TSO6 | NETIF_F_GSO_UDP_L4);
> >>>>
> >>>>  	return features;
> >>>>
> >>> Hi Paolo, Willem,
> >>>
> >>> Thank you both for the insightful analysis and the proposed fix.
> >>>
> >>> I have backported and tested Paolo's patch in our environment with hinic NIC.
> >>> We focused on the VXLAN (IPv6-in-IPv4) scenario and the Native IPv6 scenario :
> >>>
> >>> Scenario               | IPv6 Ext-Headers | Result | Behavior
> >>> -----------------------|------------------|--------|---------------
> >>> VXLAN (IPv6-in-IPv4)   | No               | PASS   | HW TSO enabled
> >>> VXLAN (IPv6-in-IPv4)   | Yes              | PASS   | SW GSO fallback
> >>> Native IPv6            | No               | PASS   | HW TSO enabled
> >>> Native IPv6            | Yes              | PASS   | SW GSO fallback
> >>>
> >>> Thanks again for the help!
> >> Please, if you will and can, take it over to cook it in a formal patch.
> > 
> > Otherwise I can.
> > 
> > The check is also needed for tunnels that set ENCAP_TYPE_IPPROTO, such
> > as sit. That condition can be removed as far as I can tell?
> > 
> > Only, I still do not see how this condition can have triggered, as
> > vlan_get_protocol(skb) should be htons(ETH_P_IP).
> > 
> > I built a simple reproducer using vxlan over veth in virtme-ng, while
> > changing veth's NETIF_F_.._CSUM to reach this code. That indeed shows
> > correct ETH_P_IP.
> > 
> > Tangxin, can you show a stack trace when this condition hits? For
> > instance by adding a WARN_ON_ONCE(1) inside that branch, or by using
> > bpftrace:
> > 
> > sudo bpftrace -e 'kfunc:netif_skb_features { if (args->skb->encapsulation && args->skb->protocol == 0xDD86) { @[kstack] = count(); } }'
> Hi Willem,
> 
> Sorry for the late reply.
> 
> I have tested this on Linux 7.0-rc4
> (commit f338e77383789c0cae23ca3d48adcc5e9e137e3c).
> 
> In my VXLAN (IPv6-in-IPv4) environment,
> vlan_get_protocol(skb) indeed returns htons(ETH_P_IP) as you expected.
> However, the condition is still triggered because skb_shinfo(skb)->gso_type
> contains SKB_GSO_TCPV6 (since the inner packet is IPv6 TCP).
>
> Below is the call trace captured via WARN_ON_ONCE when the condition is hit:
> 
> WARNING: net/core/dev.c:3824 at gso_features_check+0xbc/0x158, CPU#10: python3/16193
> CPU: 10 UID: 0 PID: 16193 Comm: python3 Kdump: loaded Not tainted 7.0.0-rc4+ #12 PREEMPT
> Hardware name: Huawei TaiShan 200 (Model 2280)/BC82AMDD, BIOS 1.86 01/10/2022
> pstate: 80400009 (Nzcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> pc : gso_features_check (net/core/dev.c:3824 (discriminator 1))
> lr : netif_skb_features (net/core/dev.c:3837)
> ...
> Call trace:
>  gso_features_check (net/core/dev.c:3824 (discriminator 1))
>  netif_skb_features (net/core/dev.c:3837)
>  validate_xmit_skb (net/core/dev.c:4012)
>  validate_xmit_skb_list (net/core/dev.c:4075)
>  sch_direct_xmit (net/sched/sch_generic.c:335)
>  __dev_xmit_skb (net/core/dev.c:4255 (discriminator 4))
>  __dev_queue_xmit (net/core/dev.c:4804)
>  neigh_hh_output (./include/net/neighbour.h:541)
>  ip_finish_output2 (./include/net/neighbour.h:554 net/ipv4/ip_output.c:237)
>  __ip_finish_output (net/ipv4/ip_output.c:315 net/ipv4/ip_output.c:297)
>  ip_finish_output (net/ipv4/ip_output.c:325)
>  ip_output (./include/linux/netfilter.h:307 net/ipv4/ip_output.c:438)
>  ip_local_out (net/ipv4/ip_output.c:134)
>  iptunnel_xmit (net/ipv4/ip_tunnel_core.c:99 (discriminator 4))
>  udp_tunnel_xmit_skb (net/ipv4/udp_tunnel_core.c:195) [udp_tunnel]
>  vxlan_xmit_one (drivers/net/vxlan/vxlan_core.c:2544) [vxlan]
>  vxlan_xmit (drivers/net/vxlan/vxlan_core.c:2832) [vxlan]
>  dev_hard_start_xmit (net/core/dev.c:3889)
>  __dev_queue_xmit (net/core/dev.c:4836)
>  ... (TCP/IPv6 stack)
>  tcp_sendmsg (net/ipv4/tcp.c:1465)
>  inet6_sendmsg (net/ipv6/af_inet6.c:653 (discriminator 2))
> 
> look forward to your patch.

Thanks for the extra info and testing. Sent:

https://lore.kernel.org/netdev/[email-masked]/T/#u